Conventional shutter mechanisms are designed to utilize the advantageous characteristics of springs to provide drive for both the opening and closing movements of light regulating blades. These spring both provide a desirable consistency of dynamic performance as well as retain an advantageous capability for storing relatively high available energy. To store this energy, the springs are loaded as by winding and the requisite power for such winding is derived manually. In more common applications, manually derived power is delivered to the springs by a cocking hand crank or lever or through a relatively strong mechanical linkage connecting a film wind lever with the shutter mechanism.
As fully automated but highly compact cameras have been developed, the noted high energy storage capacity obtainable from manually loaded springs no longer remains available to shutter designers. For instance, the power for shutter mechanism drive necessarily must be derived from a relatively small battery located within the camera. The design of a resultant efficient low power drive shutter becomes even more involved where the device is called upon not only to automatically regulate an interval of exposure, but to provide a programmed automatic dual parameter (exposure interval and aperture) control. Further, the shutter mechanism may be called upon to function within the complex photographic cycle required of a fully automated single lens reflex camera by carrying out operations not associated with the image forming exposure of film.
A photographic cycle involving such automated reflex operation is described in U.S. Pat. No. 3,714,879 by E. H. Land, I. Blinow and V. K. Eloranta. The automated reflex operation therein requires that the camera shutter remain normally open, defining full aperture width for purposes of viewing and focusing. With the commencement of a photographic cycle, the shutter is required to fully close and remain closed while the optical path of the camera converts to an exposure mode orientation. Following such conversion, the shutter is required to carry out two parameter exposure regulation, following which the blades thereof remain closed as the components of the camera are automatically driven to alter the optical path to its initial viewing-focusing mode orientation. When the latter conversion is completed, the shutter mechanism is called upon again to reassume its open condition exhibiting maximum aperture width. To conserve energy from the limited battery power supply, current drain throughout a photographic cycle imposed from the shutter must be held to the lowest levels available. In effect, the automatic cameras must be fabricated to operate within a limited power supply profile during the course of each successive photographic cycle.
One approach to providing such shutter performance is described in a copending application for U.S. patent by I. Erlichman, entitled "Non-Cocking Springless Shutter Developing Two Parameter Exposure Regulation," Ser. No. 362,926, filed May 22, 1973, and assigned in common herewith, as well as in an application for U.S. patent by R. Kee, Ser. No. 362,664, entitled "Exposure Control System For Photographic Apparatus," filed May 22, 1973 and also assigned in common herewith. These applications describe an exposure control mechanism and related drive system in which a stepper motor is utilized to drive two coacting exposure mechanism blades about a camera optical path. The mechanism blades are configured to be directly driven from the output of the stepper motor and move when so driven in synchronism and correspondence to define a predetermined variation of aperture values over the cammera optical path. The shutter utilizes no springs and requires no cocking to impart the noted movement to the exposure mechanism blades. Further, it readily is adapted to use within involved photographic cycles requiring select optical path blockage during mode conversion procedures. Exposure regulation is provided by a control circuit utilizing a photosensitive element operating in conjunction with a secondary aperture opening formed within the exposure mechanism blades. With this arrangement, as the blades are moved to define varying apertures, corresponding aperture information is provided to the control circuit by synchronous variation of light input to a photosensitive element.
The stepper motor is driven from a closed or blocking position at the commencement of an exposure interval while scene lighting is being sensed at the photosensitive element. As the blades reach an appropriate apertural value, the control circuit commands the stepper motor to rotate in an opposite direction to terminate the exposure interval. With such an arrangement, scene light evaluation is carried out coincidently with stepper motor activation. Inasmuch as stepper motors are typically constructed having multiple stator phases in conjunction with a multiple pole rotor construction, they may be halted at predetermined magnetic detent orientations, each such orientation serving to position the exposure mechanism blades at a particular location defining a predetermined aperture value. In effect, the exposure mechanism blades may be driven through a series of steps or potential halting positions. The presence of such detent positions as well as the simplicity and substantial low mass of the system, allows for very high accuracy in blade positioning.